Field of the Invention
The present invention relates to an optical scanning apparatus, and particularly to a beam position control apparatus in an apparatus for scanning a plurality of beams in parallel by use of a plurality of light sources.
Description of the Related Art
In an optical scanning apparatus such as a laser printer, light-beam scanning is realized by moving a rotary polyhedron mirror or a vibratory mirror at a high speed. In this case, as the speed and resolution of the apparatus become higher, it is indeed necessary to make its operation speed extremely higher, while there is a limitation therein.
In order to cope with this problem, an optical scanning apparatus in which a plurality of light beams are scanned in parallel at the same time by use of a plurality of light sources where the intervals between the respective beams are kept constant has been proposed, such as a laser printer optics using two semiconductor lasers, in FIG. 12 in Japanese Unexamined Patent Publication No. Sho-63-217763 as well as in FIG. 7.63 in "Applied Optical Electronics Handbook" (published by Shokodo Co., Ltd., Apr. 10, 1989). FIG. 8 shows an example of such an optical scanning apparatus. In this example, it is necessary to provide a controller 1 for attaining the above-mentioned object, and, generally, the controller is arranged so as to perform servo control. This servo control technique is disclosed in the above-mentioned Applied Optical Electronics Handbook. That is, in Chapter 6, "Optical Information Equipment Elementary Techniques," in the Handbook, FIG. 6.15 shows a block diagram of an optical disk system servo system, FIG. 6.138 shows a block diagram of a focusing servo system, FIG. 6.139 shows an open loop characteristic of a focusing servo system, etc. However, these systems belong to a control technique for an optical disk system, and the following problems to be solved occur in the case of controlling such "laser printer optics using two semiconductor lasers" as shown in FIG. 8.
In FIG. 8, when the controller 1 receives a signal 3 from a light detector 2 to thereby control movable reflectors 4a and 4b, both beams 5a and 5b must be partially incident to the light detector 2. In order to make the beams 5a and 5b be within the light detector 2 when the electric power source of a laser printer or the like is turned on, however, it is necessary to adjust initial angles of the movable reflectors 4a and 4b so as to set initial positions of the beams accurately. There is however a problem, as this adjustment is complicated and the setting becomes easily displaced by the disturbance such as a temperature change, vibrations, etc.
In addition, when the beams are impinging onto a drum surface 6, that is, when printing is being performed, the beams also impinge onto the light detector 2 so as to make servo control possible, but when there is no print signal, that is, at the off-time of light generating sources 7a and 7b in the duration in which they are repeatedly turned on and off, there is a problem that the beams do not impinge onto the light detector 2. In order to solve this problem, a sample holding circuit which obtains a sample signal by turning on the light generating sources in the non-printing period and holds the sample signal in the other period is provided (for example as disclosed in Japanese Unexamined Patent Publication No. Sho-61-245174). In this technique, however, there is a problem of control delay.
In addition, a beam interval P.sub.D on the drum surface 6 depends on a beam interval P on the detector 2, and generally the detector 2 is constituted by four optical sensors 10 to 13 as shown in FIG. 9. The beam 5a is controlled to be in the center of the optical sensors 10 and 11, and the bean 5b is controlled in the center of the optical sensors 12 and 13, so that the beam interval P on the light detector 2 is kept constant. In this case, although the beam interval P is set into a predetermined value, there is a problem that setting into the predetermined value is difficult due to either the scattering of positions and characteristics on producing the optical sensors 10 to 13 or the beam shape errors (for example, not a true circle) of the beams 5a and 5b.
Alternatively, there is a case in which galvano-mirrors having as high of a reliance as those used for tracking actuators in an optical disk apparatus are used as the movable reflectors 4a and 4b. In this case, the movable reflectors 4a and 4b have a moving coil type response and thereby become secondary delay factors. In order to stabilize this time control system, as shown in FIG. 10, it is common to perform phase adjustments to take a large enough value for a gain G, for example, a gain surplus of 17, at a frequency f which makes a phase ph be -180.degree. as described above (the open loop characteristic of a focusing servo system). In this case, the initial angles of the movable reflectors 4a and 4b of FIG. 8 are set so that the beams 5a and 5b come in the center of the optical sensors 10 and 11 and in the center of the optical sensors 12 and 13, respectively, as shown in FIG. 9.
Assume that the beam 5a or 5b comes away by .OMEGA. from the center of the optical sensors 10 and 11 or from the center of the optical sensors 12 and 13 in FIG. 9. Then, if the servo control of FIG. 10 is performed, the center of the beam 5a or 5b comes away by (1/1+G.sub.o).times..OMEGA. from the center of the optical sensors 10 and 11 or from the center of the optical sensors 12 and 13, wherein G.sub.o represents a DC gain in FIG. 10. In order to establish the beam interval P.sub.D of the optical scanning apparatus in FIG. 8 to be a value of the order of several .mu.m, it is necessary that the DC gain G.sub.o takes a large enough value. However, since the DC gain G.sub.o depends on the characteristics of the movable reflectors 4a and 4b, the gain surplus 17 is lowered if the gain G is increased, causing the problem that the control becomes unstable.